In recent years, ozone has been used for a variety of purposes such as cleaning of a silicon wafer used in a semiconductor device, water supply and sewerage treatment, and the like. In cleaning of a silicon wafer, ozone water in which ozone gas is dissolved in pure water is used as a cleaning solution, and hence an organic substance or noble metal on a silicon wafer is removed by using the ozone water together with dilute hydrofluoric acid aqueous solution or the like.
It is difficult to store ozone gas in a state of gas due to the strong self-decomposition property thereof, and ozone gas is generally generated for use by an ozone generator in the vicinity of an ozone-using facility.
A conventional method of concentrating ozone includes an adsorption step of allowing an adsorbent cooled by a refrigerating machine to saturation-adsorb ozone gas generated by an ozone generator in an atmospheric state, a purification step of discharging, under a reduced pressure, an adsorption pipe containing the adsorbent to a partial pressure of supply ozone gas, and an elimination step of supplying ozone of high concentration with a pressure difference by communicating the ozone to an ozone-using facility kept in a vacuum state, while keeping a cooled state and a reduced-pressure state in the adsorption pipe (for example, see Patent Document 1).
Further, as another conventional method of concentrating ozone, the following has been proposed: an apparatus includes three adsorption towers filled with an adsorbent, a cooling device for cooling the adsorption towers, a heater for controlling the temperature of the adsorption towers, an open/close valve for changing an ozone passage, an ozone storage container for storing desorbed ozone, and a mass-flow controller for adjusting the flow rate of ozone to be supplied from the ozone storage container to an ozone-using facility, and four steps of an ozone adsorption step, a stabilizing and boosting step, an ozone desorption step, and a cooling step are repeated and the operation of each adsorption/desorption column is shifted by ⅓ cycle to generate ozone of a concentration of 28.4 vol % continuously (for example, see Patent Document 2).
Further, in a conventional discharge-type ozone generator, several % of nitrogen is mixed in oxygen gas to stabilize the generation of ozone (for example, see Patent Document 3).    Patent Document 1: JP 2002-68712 A    Patent Document 2: JP 11-335102 A    Patent Document 3: JP 2001-180915 A